While some PhD students might like to think of them selves as a young Newton or Curie in the making, for me it’s more like Johnny Cash. Except I walk the graphite line rather a moral one. To turn a sample into something that you can actually radiocarbon date it needs to be in a form that can be introduced to the accelerator mass spectrometer, or AMS. First, the sample is combusted inside a sealed quartz tube (or for carbonate samples CO2 is evolved by adding acid to the sample inside a sealed blood vial). The sample is then introduced to the graphite line (pictured above) where it is passed over a water trap, frozen down with liquid nitrogen to evacuate any non-condensible gases, and then fed through to individual reactors. Each reactor has iron catalyst inside and when heated above 450C will produce filamentous graphite, or iron carbide from the CO2 from your sample. This iron carbide is then pressed into a target and popped into a wheel to be placed in the accelerator. Easy peasy I hear you say.
It wasn’t always easy peasy though. Look how many valves there are! Do you have any idea how easy it is to open the wrong valve? You can open the valve to the vacuum and your sample is sucked outside in an instant (done that), or open the wrong valve and combine two samples rendering them both useless (done that), you can forget to evacuate the bellows and mix atmospheric CO2 with your sample (done that) and then you can forget which part of the line the sample is in and freeze with terror not wanting to open any valves for fear of any of the above (done that more times than I like to admit). The first few times I walked the line the process of preparing 12 samples took me 3 hours, atleast now it is down to just over an hour. Every time you move the sample you wait. You wait for it to freeze, to defrost, to pump down and to equilibrate. It is tempting to try to accomplish other tasks while waiting, but then you get confused about where you are up to in the process and are more likely to open the wrong valve….again. It’s like labelling, it’s monotonous, and often a little soul destroying, but it is oh so important that you get it right. So, for the 15 millionth time in the last 4 years, while the rest of Australia enjoyed a public holiday, I walked the line. (And this is where my similarity with Johnny Cash ends.)